Gifford McMahon (GM), GM type pulse tube, Stirling and Stirling type pulse tube refrigerators operating in a MRI cryostat are used to cool a superconducting magnet either by conduction through a direct connection between the cold end of the expander and the magnet, or by operating in a helium filled neck tube of the MRI cryostat where the refrigerator recondenses helium that is cooling the MRI magnet. Both a conventional type GM expander, as described in U.S. Pat. No. 5,447,033 by Nagao, and a pulse tube type as described in U.S. Pat. No. 6,256,998 by Gao, use rare earth materials in the cold end of the regenerator in order to provide cooling at 4 K.
The rare earth materials have magnetic properties that cause them to interact with the magnetic field in the bore of the MRI magnet and result in noise being superimposed on the imaging signal. This problem is described in JP 2600869 by Nagao, filed on 13 Dec. 1988, for a conventional GM refrigerator in which the rare earth material is contained in the reciprocating second stage displacer. This patent describes several different designs of magnetic shields that prevent the rare earth material from introducing noise into the MRI signal. The expanders are described as operating in the vacuum in the MRI cryostat; thus the magnetic shield can be attached to an expander heat station and cooled to a uniform temperature without heat transfer between the shield and the expander cylinder.
Means for constructing superconducting shields are described in Bogner, U.S. Pat. No. 3,331,041, filed on 21 Apr. 1968, and Saji, U.S. Pat. No. 4,803,452, filed 29 Dec. 1987. More recently, high temperature superconductors (HTS) have been developed that operate at temperatures in the range of 80 K. While the earlier shields were designed for operating temperatures near 4 K, it is now possible to construct shields from HTS materials and cool them at the first stage of a two stage GM type refrigerator.
Eckels, U.S. Pat. No. 5,701,744, issued 30 Dec. 1997, describes a conventional two stage 4 K GM expander mounted in the neck tube of a MRI cryostat where it is surrounded by helium gas. The expander plugs into the neck tube and can easily be removed to be serviced without warming the magnet. It comes into thermal contact with a first stage heat station in the neck tube that is connected to a thermal shield at about 40 K, and into thermal contact at the second stage 4 K heat station which is connected to a helium recondenser and a magnetic shield.
The magnetic shield is located on the vacuum side of the neck tube so that it is thermally isolated from the neck tube and the expander cylinder. Thermal conducting strips are layered with the superconducting shield material to keep the assembly near 4 K.
Two-stage GM type pulse tubes operating at 4 K introduce much less noise to a MRI signal than conventional two-stage GM expanders because the cold regenerator containing rare earth materials is stationary. However noise continues to be introduced either due to small motion from pressure cycling the pulse tube, or due to thermal cycling of the regenerator material.
Most conventional GM expanders have the regenerators packed inside the displacer bodies, thus they reciprocate with the displacer. This arrangement provides a compact design with a single stepped cylinder. When mounted in the neck tube of a MRI cryostat, the helium gas in the neck tube transfers heat by convection between the neck tube and the expander cylinder. Thermal losses are minimized by mounting the neck tube in a near vertical orientation. The expander cylinder and the neck tube have almost the same temperature gradients.
Pulse tube expanders typically have the regenerators in tubes that are spaced closely apart from the pulse tubes and parallel to them. When mounted in a MRI neck tube surrounded by helium gas the temperature differences between the regenerators and pulse tubes result in convective thermal losses between them, in addition to thermal losses due to heat exchange with the neck tube.
U.S. Ser. No. 60/650,286 entitled “Multi-stage Pulse Tube with Matched Temperature Profiles” filed Feb. 4, 2005, incorporated herein by reference and made a part hereof, describes means to minimize thermal losses due to convection in a helium filled neck tube. The addition of a magnetic shield, as described in JP 2600869, within the neck tube is impractical because a shield that uses low temperature superconducting material has to be kept near 4 K, thus convective losses would be excessive. If the magnetic shield is placed in the vacuum outside the neck tube, as described in U.S. Pat. No. 5,701,744, it becomes large and expensive.
Stirling type pulse tubes have recently been built with magnetic materials in the regenerators and have reached 4K. This has been difficult to achieve because of their relatively high operating speeds.
In the prior art, the magnet shield is attached at the outside of the cylinder, which requires a large size shield. Typically, the shield is made of superconducting materials, which are expensive especially in a pulse tube refrigerator, where the 2nd stage heat station is much larger than that of a GM refrigerator.
Therefore, it is an objective of the present invention to provide a magnetic shield which effectively shields the MRI equipment from magnetic materials in the expander.
It is an object of this invention to provide a magnetic shield which is in a vacuum enclosure that is part of the expander assembly, and is independent of the vacuum in the cryostat.
It is a further object of this invention to provide a magnetic shield which effectively shields the MRI equipment from magnetic materials in the expander while minimizing heat losses.
It is a further object of this invention to provide a magnetic shield which can be used with Stirling refrigerators and Stirling type pulse tubes.
It is another object of this invention to provide a regenerative type expander where magnetic material is surrounded by a magnetic shield that may be fixed or removable for easy servicing.
These and objects will be apparent from the following description.